This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-232682, filed Aug. 19, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a position control apparatus for controlling the position of a movable part, for example, a movable mold, driven by a servomotor in a motor-driven injection molding machine. The position is controlled by monitoring the present position of the movable mold, the positional error between the present position and the target position and the rotation rate of the servomotor.
In a conventional motor-driven injection molding machine, the position of a movable part driven by a servomotor is controlled by supplying to the servomotor a current periodically output in accordance with a command position.
According to the conventional method, however, there may be a positional error between the present position of the movable part and the target position, if a product is jammed between the movable part and a fixed part or if the servomotor cannot rotate due to a high load relative to a low torque command. To compensate for the positional error, position command values are successively renewed and output. The renewed position command values have commands for gradually increasing the drive current of the servomotor.
If the above state lasts long, an overcurrent flows through the servomotor, resulting in burning. To prevent this, the control unit incorporated in the motor-driven injection molding machine determines that an error occurs, so that the operation of the machine is stopped. Once the operation is stopped, half-produced products are disposed of as inferior goods. In addition, a number of procedures are required to restart the operation, resulting in reduction of the yield or the operation efficiency. Further, since a large current flows through the servomotor, if the jammed product happens to remove during the operation, the servomotor will be rotated at a very high speed. Therefore, the operator may be exposed to danger and the machine may be damaged.
Accordingly, an object of the present invention is to provide a position control apparatus which does not allow an overcurrent to flow through a servomotor, if the servomotor is locked due to product jam, etc., so that reduction in yield and operation efficiency can be prevented, no danger may be posed to the operator, and damage to the machine can be avoided.
According to the present invention, the above object is achieved as follows: when the servomotor is unable to rotate because of product jam or the like in an apparatus to be controlled, a position control process prohibits a position command value from being renewed, so that positional errors may not be accumulated.
A position control apparatus of the present invention comprises: a servomotor for driving a movable part of a motor-driven injection molding machine; position command means for renewing a position command value of the movable part, output in a predetermined cycle in accordance with a positional error between a present position and a target position of the movable part; limiting means for limiting a current value supplied to the servomotor, when the positional error is equal to or greater than a predetermined value or a rate of the servomotor is equal to or smaller than a predetermined value; and means for stopping a renewal of the position command value or extending a renewal cycle thereof, when the current value is limited.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.